1. Field of the Invention
The present invention relates to a display device including a spatial light modulator such as a liquid crystal display.
2. Description of Related Art
A liquid crystal display (LCD) is a well known example of display devices.
As shown in FIG. 1, a conventional display device 300 includes: a fluorescent lamp 302, a liquid crystal display 304, and an imaging lens 306. The fluorescent lamp 302 serves as a surface light source. The liquid crystal display 304 modulates light from the fluorescent lamp 302 to selectively transmit the light therethrough. The imaging lens 306 receives the light from the liquid crystal display 304 and focuses the light onto a retina 310 in the eyeball 308 of a user, thereby producing an image on the retina 310. The display device 300 with this configuration allows the user to view the image formed from the light modulated by the liquid crystal display 304.
However, the display device 300 of FIG. 1 has the following problems.
First, because the fluorescent lamp 302 is a type of light source that diffuses light, a focal depth where the image is into focus is shallow. Therefore, the image likely becomes out of focus and becomes fuzzy.
More specifically, it is now assumed that an imaginary stop or diaphragm 314 is located in front of a single point on the liquid crystal display 304 as shown in FIG. 1 for explanation purposes. The imaginary stop 314 is not actually provided. Light emitted from the entire region of the fluorescent lamp 302 falls incident on the single point of the liquid crystal display 304 at an angle of incidence xcex81 after passing through the imaginary stop 314. For this reason, the incident angle xcex81 becomes large. Then, the light emits from the same point of the liquid crystal display 304 at an angle of emission xcex82 which is substantially equal to the incident angle xcex81. The emission angle xcex82 is therefore also large. Accordingly, only a portion of the entire emission light will fall incident on the imaging lens 306. More specifically, in the entire emission light within the emission angle xcex82, only the portion of the light within an angle xcex83 falls incident on the imaging lens 306. After passing through the imaging lens 306, only a portion d1 of the light falls incident on the pupil of a user""s eyeball. The pupil is surrounded by an iris 316. After entering the pupil, the light finally reaches the retina 310 after passing through a crystalline lens 318. In this way, light emitted from the single point on the liquid crystal display 304 focuses onto the retina 310 at a relatively large angle xcex84 which has a size corresponding to the almost entire region of the crystalline lens 318. Accordingly, the focal depth of the light that forms an image on the retina 310 is shallow, so if the focus 320 shifts slightly forward or behind the retina 310, the image will appear out of focus and fuzzy.
Also, with this configuration, light is emitted from the single point of the liquid crystal display 304 in the large emission angle xcex82. Therefore, when the liquid crystal display 304 is exposed to the outside, the image displayed on the liquid crystal display 304 can be seen by people other than the user, so that the displayed image cannot be kept secret or confidential.
Also, the fluorescent lamp 320 consumes a great deal of power. This is a particularly problem when the display device 300 is used as a part of a portable device because the high power consumption quickly drains batteries.
In view of the above-described drawbacks, it is an objective of the present invention to provide an improved display device with an increased range of focal depth of light forming images on a retina so that images almost never appear out of focus.
It is another objective of the present invention to provide an improved display device with high degree of confidentiality so that people other than the user cannot see the image formed by the spatial light modulator.
It is still a further objective of the present invention to provide an improved display device with low power consumption and capable of suppressing the consumption of butteries when assembled as a part of a portable machine.
In order to attain the above and other objects, the present invention provides a display device, comprising: a point light source that emits white light; a spatial light modulator spatially modulating the light emitted from the point light source to produce an imaging light; and an optical imaging system receiving the imaging light modulated by the spatial light modulator and producing an image.
Because the display device employs the point light source, the spatial light modulator emits the modulated imaging light at a relatively small angle of emission. Accordingly, even when a user""s eye slightly moves along the optical axis of the display device, the image on the user""s eye will not become out of focus and will not appear blurry.
The display device may preferably further comprise an optical light-collecting system gathering the light from the point light source, the spatial light modulator receiving the light collected by the optical light-collecting system and spatially modulating the light to produce the imaging light.
With the structure, light emitted from the point light source is collected by the optical light-collecting system, such as a condenser lens, and reaches the spatial light modulator. After being modulated by the spatial light modulator, the light emits from the spatial light modulator at a relatively small angle of emission. As a result, the light enters a user""s eyeball at a small angle of convergence. Accordingly, the light focuses on the user""s retina with a deep focal depth. Even when the user""s retina slightly moves along the optical axis of the display device, the image formed on the user""s retina will not be out of focus.
Additionally, because the light emits the spatial light modulator at the relatively small emission angle, the image formed by the light will not be seen by someone other than the user who is situated at a position behind the optical imaging system.
The optical imaging system may preferably be located to cause the point light source and a first position, which is located behind the optical imaging system and which is separated from the optical imaging system by a desired distance, to have substantially a conjugate relationship and to cause the spatial light modulator and a second position, which is located behind the first position and which is separated from the first position by a distance substantially equal to a distance between a pupil and a retina of a user""s eye, to have substantially a conjugate relationship.
When the point light source and the first position have the substantial conjugate relationship, the almost entire part of the light that has passed through the optical light-collecting system can be guided into the user""s eyeball through the pupil. Accordingly, the point light source needs only to have a smaller light emitting power in order to irradiate the interior of the pupil with a fixed amount of optical power. It is therefore possible to suppress the power consumed by the point light source. When the spatial light modulator and the second position have the substantial conjugate relationship, light modulated by the spatial light modulator can be observed at the second position.
The display device may preferably further include a scattering plate located between the point light source and the spatial light modulator. In comparison with the case where no scattering plate is provided, a light bundle emitted from a single point of the spatial light modulator will spread in a relatively wide range at a position in the vicinity of the user""s pupil, for example. Accordingly, it is ensured that the light bundle can be guided into the user""s eyeball even when the user""s eyeball slightly shifts in a direction normal to the optical axis of the display device. There will be little variation in the intensity of the formed image.
Also when the scattering plate is thus additionally provided in the display device, the optical imaging system may preferably be located to cause the point light source and a first position, which is located behind the optical imaging system and which is separated from the optical imaging system by a desired distance, to have substantially a conjugate relationship and to cause the spatial light modulator and a second position, which is located behind the first position and which is separated from the first position by a distance substantially equal to a distance between a pupil and a retina of a user""s eye, to have substantially a conjugate relationship.
When the point light source and the first position have the substantial conjugate relationship, regardless of the scattering plate, a large part of the light that has passed through the optical light-collecting system can be guided into the user""s eyeball through the pupil. Accordingly, the point light source needs only to have a smaller light emitting power in order to irradiate the interior of the pupil with a fixed amount of optical power. It is therefore possible to suppress the power consumed by the point light source. When the spatial light modulator and the second position have the substantial conjugate relationship, light modulated by the spatial light modulator can be observed at the second position.
The display device may further comprise a holding portion that enables a user to transport the casing. For example, the holding portion may include a portion for being supported by at least one hand of a user. Or, the holding portion may include a portion for being attached to a user""s head. The holding portion may include a portion for being attached to a frame of a pair of glasses. The display device may further comprise an image pick up device for picking up an image of the user""s face.